1. Field of the Invention
The present invention relates to an active matrix substrate inspecting device for uncovering faults in an active matrix substrate and a method of inspecting an active matrix substrate for the presence of a fault.
2. Description of the Prior Art
A liquid crystal display device of an active matrix system includes an active matrix substrate which comprises pixel electrodes disposed on an insulating substrate in a matrix and driving elements such as a thin film transistor (hereinafter referred to as "TFT") connected to each pixel electrode. The driving element includes a two-terminal element, a three-terminal element, or the like.
As an example of the two-terminal element an equivalent circuit such as a diode ring, a back-to-diode, and a metal-insulating layer-metal (MTM) is shown in FIG. 8. This active matrix display includes a plurality of signal lines 31 and a plurality of signal scanning lines 35 crossing the signal lines 31. In each area which is encompassed by the scanning line 35 and the signal line 31, a two-terminal element 32 and a pixel capacitance 37 are disposed. This active matrix display device is so constructed that a liquid crystal layer 34 (FIG. 8) is interposed between an active matrix substrate shown in FIG. 9A and a counter substrate shown in FIG. 9B. In the active matrix substrate shown in FIG. 9A, the signal line 31 and a two-terminal element 32 are provided. One terminal of the two-terminal element 32 is connected to the signal line 31, and the other terminal thereof is connected to the pixel capacitance 37 which has a pixel electrode 33 on the active matrix substrate (FIG. 9A), a counter electrode 36 on the counter substrate (FIG. 9B), and the liquid crystal layer 34 sandwiched therebetween. The pixel capacitance 37 stores electric charge, thereby modulating the optical characteristic of the pixel electrodes.
Another example using a three-terminal element is shown in the form of an equivalent circuit using TFTs in FIG. 10. This display device consists of an active matrix substrate 45 shown in FIG. 11, a counter substrate (not shown) formed of a transparent conductive layer on the entire surface of an insulating substrate, and a liquid crystal layer 34 sandwiched between the active matrix substrate 45 and the counter electrode. Likewise, the scanning lines 35 and the signal lines 31 cross each other in an insulated state on the active matrix substrate 45. A gate electrode 41 of the TFT 40 is connected to the scanning line 35. The source electrode 42 of the TFT 40 is connected to the signal line 31. The pixel electrode 33 is connected to a drain electrode 43 of the TFT 40.
This display device is operated in the following manner:
Initially, a single scanning line is selected from the scanning lines 35. Then, an on-voltage is applied to the gate electrode 41, thereby outputting video signals simultaneously or sequentially to a plurality of the signal lines 31. The output signals are stored in the pixel capacitance 37 via the source electrode 42 and the drain electrode 43 of the TFT 40. The video signals stored in the pixel capacitance 37 is held during a one-frame period till the next on-voltage is applied after the voltage of the scanning line 35 is off. The voltage in the form of video signals changes the orientation of molecules of the liquid crystal layer 34 in the pixel capacitance 37 and performs a display. To enhance the holding characteristic of the video signal, an additional capacitance is often provided in parallel with the pixel capacitance 37. In addition, in the case of color display, a color filter is provided to each pixel.
An active matrix substrate of the active matrix display device is liable to fault during the production process because numberless driving elements and pixel electrodes are formed on the active matrix substrate. In general, two types of faults occur in the active matrix substrate; that is, a linear fault and a dot fault. Some linear faults are caused by the breakage of the scanning line 35 or the signal line 31, a leakage occurring on the scanning line 35 or the signal line 31, and the leakage between the scanning line 35 and the signal line 31. Dot faults are caused by non-activation of on-state or off-state of the active element and a leakage occurring between the pixel electrode 33, the scanning line 35, and the signal line 31.
Inspection for these faults must be made during the production process. Inspection for a linear fault can be made when an active matrix substrate is produced. More specifically, any linear fault is inspected by a short-circuit check between the scanning lines 35, between the signal lines 31, and between the scanning line 35 and the signal line 31.
In the case of TFTs in an active matrix substrate, faults are detected by using probes which are stuck in the pixel electrodes 33 and applying voltage to the scanning line 35 and the signal line 31. This inspecting method, disadvantageously, takes time. In addition, the active matrix substrate is likely to be damaged by the probes. This inspecting method cannot be applied to all pixels.
As described above, the known fault inspecting method is performed on an assembled display device because of the ease and accuracy for finding faults as compared with when it is performed on the active matrix substrate in a disassembled state. If the fault inspection is performed on the active matrix substrate, cost expenditure will be reduced on material such as an orientation film, liquid crystal, a counter substrate, and a color filter on the counter substrate. However, as described above, the active matrix substrate of the above-mentioned structure does not allow a fault inspection to be conducted directly on the active matrix substrate.